1. Field of the Invention
This invention relates generally to scintillation detector systems and, more particularly, to scintillation material having improved stability at continuously high operating temperatures.
2. Discussion of Related Art
Nuclear radiation has been used for borehole and well analysis, generally referred to as logging. Detecting and measuring radiation permits an evaluation of the properties of a formation surrounding the borehole and therefore, is used for locating and extracting, for example, radioactive mineral deposits and petroleum.
As illustrated in commonly assigned, U.S. Pat. No. 5,293,410, issued Mar. 8, 1994, to Felix K. Chen et al., the disclosure of which is incorporated by reference herein in its entirety, high energy neutron generators are particularly useful in well logging applications. In such applications one important factor is accurate knowledge of the neutron pulses that irradiate the surrounding formation. For example, it is desirable to accurately measure the neutron output (e.g., number of neutrons emitted by the neutron detector). As described by Chen et al., ensuring stable generator output is important for successful implementation of neutron generators in logging applications.
Logging tools typically employ radiation detectors to assist in control of neutron output. Chen et al. and commonly-assigned U.S. Pat. No. 5,539,225, issued Jul. 23, 1996, to William A. Loomis et al., the disclosure of which is hereby incorporated by reference in its entirety, describe drilling apparatus that include neutron detectors. For example, Loomis et al. describe a drill string having a neutron generator, at least one neutron detector (e.g., a scintillation detector), a photodetector optically coupled to the scintillation detector and supporting electronic systems. These components cooperate to process neutron bombardment of the detector such that information is obtained regarding the neutron output of the generator. Such information is provided to control devices for assisting in stable neutron generation. Commonly-assigned U.S. Pat. No. 4,760,252, issued Jul. 26, 1988 to Paul Albats et al., and U.S. Pat. No. 4,972,082, issued Nov. 20, 1990 to William A. Loomis, et al., also incorporated by reference in their entireties, disclose wireline applications having similar neutron detectors.
FIG. 1 illustrates a conventional scintillation detector, shown generally at 10. The scintillation detector 10 is comprised of two basic components. Scintillation material 12 efficiently interacts with incident radiation (e.g., emitted by a neutron generator). The incident energy excites fluorescent materials contained in the scintillation material 12 such that the fluorescent materials give off light (e.g., scintillation light). A second component is a light detector such as a photodetector 14 that converts the scintillation light into an electronic signal. The electronic signal is processed by systems, such as described in Loomis et al., to obtain information related to the neutron output of the neutron generator.
In a conventional logging apparatus, scintillation material is comprised of a specially formulated organic polymer or plastic. The inventors have noted that many conventional plastic scintillators generally do not exhibit acceptable mechanical and optical properties when used at relatively high temperature (in excess of 75° C. and up to, for example, 175° C.) encountered in a borehole. In spite of this perceived deficiency, development of a plastic scintillator for well logging is preferred as plastic scintillators are particularly useful in selectively detecting fast neutrons (e.g., neutrons above about 0.5 MeV energy) that are produced from accelerator based neutron generator in drill strings and sondes. Accelerator based well logging tools are perceived as a very effective and safe means for evaluation of geological formation properties. The inventors have observed that these properties are more exactly quantified when the fast neutron flux is measured during tool operation. Fast neutrons are selectively detected when the neutron impacts a hydrogen atom nucleus in the scintillator. As is generally known, momentum transfer is very efficient for this type of collision since the neutron and proton masses are almost identical. A recoil proton transfers its energy to the hydrogenous plastic matrix by means its electrostatic field. The sensitivity to other types of nuclear radiation is lower and so a high signal-to-noise ratio is achieved for fast neutrons.
Commonly owned, U.S. Pat. No. 4,578,213, issued Mar. 25, 1986 and U.S. Pat. No. 4,713,198, issued Dec. 15, 1987, to John J. Simonetti (the Simonetti Patents), the disclosures of which are incorporated by reference herein in their entireties, each describe a high temperature plastic scintillator comprising a polymethylpentene (PMP) thermoplastic material containing a fluorescent additive. The plastic scintillator is described as maintaining excellent optical properties for detecting neutrons at high temperatures, e.g., as high as 200° C. The Simonetti Patents describe that the PMP thermoplastic material is an improvement over conventional thermosetting plastics such as polystyrene, polyvinyl toluene and various acrylic polymers.
As illustrated by the aforementioned Simonetti Patents, plastic scintillator materials have been available commercially for many years. A comparison of some of the scintillator materials and observed properties is tabulated below in Table 1.
TABLE 1Melting/SofteningEmis-H/CDensityPointLight OutputsionRatioScintillator(g/cc)(Tg) ° C.(% Anthracene)(nm)(atomic)THERMOSCIN0.835230184202.20BC-4381.03275554251.00
BC-438, a scintillator manufactured by Bicron Business Unit of Saint-Gobain Industrial Ceramics, Inc., Newbury, Ohio, USA, employs polyvinyltoluene (PVT) for a host plastic material. It is believed that the PVT is cross-linked to improve the materials resistance to deformation above room temperature. However, the host polymer has a melting point of only about 75° C. and cannot withstand borehole temperatures. For example, U.S. Pat. No. 4,833,320, issued May 23, 1989, to Charles R. Hurlbut, describes a plastic scintillation element of a scintillation detector that softens and deforms at high operating temperatures (e.g., in borehole applications). Hurlbut describes enclosing the scintillation material in a metal retaining cup for attempting to hold the scintillation material in its original shape during high temperature operation.
The assignee of the present invention has modified PMP, generally considered a high temperature host polymer by those skilled in the art, to provide scintillator properties. The modified PMP is manufactured under the brand name THERMOSCIN. As illustrated in Table 1, the THERMOSCIN is temperature resistant and has a relatively high hydrogen content as compared to conventional scintillation material as illustrated by the BC-438 sample of Table 1. However, the modified PMP is seen to offer low efficiency for generating scintillation pulses. One perceived reason for the low light output is that the PMP host polymer does not have an extended system of π (pi) electronic bonding. This type of organic carbon-carbon bonding is arranged in some molecular forms to be highly delocalized. The delocalization or aromatic character is seen as a key factor for distribution of energy from one location in a polymer to another. PVT based scintillators such as, for example, the aforementioned BC-438 scintillators, have this aromatic character and as a result demonstrate a high light output or high energy transfer efficiency.
U.S. Pat. No. 4,127,499, issued Nov. 28, 1978, to Tseng J. Cheng et al. describe a thin film scintillator composition formed from a latex dispersion manufacturing process. The thin film composition includes fluor concentrations described as providing counting efficiencies for detecting radiation emissions as low as 0.01 MeV. Chen et al. further describe a preferred embodiment where the thin film composition is used as a coating on a photographic support (e.g., glass, metal, film or paper supports) having a coating coverage of 5 to 40 milliliters/100 cm2, and more preferably 10 to 30 ml/100 cm2. A perceived disadvantage of the thin film scintillation materials of Chen et al. is that it is not suited for high temperature operation as experienced in borehole environments.
Accordingly, the inventors have realized that a need exists for improved host polymers that exhibit stable properties at relatively high temperatures (e.g., in excess of at least 175° C.) while also exhibiting improved efficiency for generating scintillation pulses.